Electric current control method for a servomotor

ABSTRACT

An electric current control method for a servomotor using DQ conversion capable of increasing an acceleration torque and stabilizing a current control for deceleration when a voltage is saturated. It is determined whether the voltage command is saturated over a maximum voltage which a power amplifier is able to output (Step T1). When the voltage command is saturated, a saturation process is performed for rewriting an integrator of a Q-phase current control loop (Step T2), it is further determined whether the servomotor is in acceleration or deceleration (Step T3) and a further saturation process is performed for rewriting an integrator of a D-phase current control loop (Step T4) only when it is determined that the servomotor is in deceleration.

TECHNICAL FIELD

The present invention relates to an electric current control method foran AC servomotor to be used as a drive source in a machinery such as amachining tool and an industrial machine, or a robot.

BACKGROUND ART

FIG. 10 is a block diagram showing a control system of a conventional ACservomotor. In this control system, a position feedback value detectedby an encoder, etc. is subtracted from a position command to obtain aposition deviation, and the obtained position deviation is thenmultiplied by a position gain in term 1 to obtain a speed command by aposition loop control. A speed feedback value is subtracted from thespeed command to obtain a speed deviation, and a speed loop process of aproportional-plus-integral control is performed in term 2 to obtain atorque command (current command). Further, a current feedback value issubtracted from the torque command and a current loop process isperformed in term 3 to obtain a voltage command of each phase. Based onthe voltage commands, the AC servomotor M is controlled by a PWMcontrol, etc.

In controlling a three-phase AC servomotor in the above-mentionedcontrol system, an alternating current control method for controllingcurrents of three phases individually in a current loop. In this currentcontrol method, a torque command (current command) obtained by the speedloop process is multiplied by each of sine waves which are shifted by anelectrical angle of 2π/3 for U, V and W phases, respectively from arotor position θ of the servomotor detected by the encoder, to obtain acurrent command of each phase. Then, current deviations are obtained bysubtracting actual currents Iu, Iv, Iw detected by current detectorsfrom the three current commands, respectively, and aproportion-plus-integral (PI) control for currents of the individualphases is performed to output command voltages Eu, Ev, Ew for therespective phases to the power amplifier. In the power amplifier, PWMcontrol is performed by an inverter, etc. to provide currents Iu, Iv, Iwfor the individual phases to flow in the servomotor M, thus driving theservomotor M. As a result, a current loop is formed as the innermostminor loop of the position and speed loops, and this current loopcontrols a current flowing in each phase of the AC servomotor.

In the above method for controlling the currents of the three phasesseparately, since the frequency of each current command rises as therotational speed of the motor increases to cause the gradual phase lagof the current, the reactive component of current increases to rise aproblem that torque cannot be generated with good efficiency. Also,since the controlled variable is alternating current, even in a steadystate in which the rotational speed and the load are constant,deviations such as a phase lag with respect to the command, attenuationof the amplitude, etc. occur, making it difficult to attain torquecontrol comparable to that attainable with a direct-current motor.

As a solution to the above problems, a DQ control method is known inwhich the three-phase current is converted into a two-phase, i.e., D-and Q-phase, in direct-current coordinate system through a DQconversion, and then the individual phases are controlled bydirect-current components.

FIG. 11 illustrates a control system in which an AC servomotor iscontrolled through the DQ conversion. It is assumed that the D-phasecurrent command is "0", and that the current command for Q-phase is atorque command outputted from the speed loop. In a converter 9 forconverting the three-phase current to a two-phase current, D- andQ-phase currents Id and Iq are obtained by using actual currents of u-,v- and w-phases of the motor, and the phase of the rotor detected by arotor position detector 7, and the currents thus obtained are subtractedfrom the command values of the respective phases, to obtain D- andQ-phase current deviations. In current controllers 5d and 5q, therespective current deviations are subjected to proportional and integralcontrol, to obtain d- and q-phase command voltages Vd and Vq,respectively. Another converter 8 for converting the two-phase voltageto a three-phase voltage, obtains u-, v- and w-phase command voltagesVu, Vv and Vw from the two-phase command voltages Vd and Vq, and outputsthe obtained command voltages to a power amplifier 6, whereby currentsIu, Iv and Iw are fed to the respective phases of the servomotor bymeans of inverters etc. to control the servomotor.

Generally, the D-phase and Q-phase voltages Vd, Vq converted by DQconversion can be expressed by the following equation (1): ##EQU1##

Therefore, ##EQU2##

Now assuming that R+sL=Z, the following equations (1)' are obtained.##EQU3##

Adopting a direct-current control method by DQ conversion, it ispossible to reduce a usual deviation without setting a current loop gainin an unnecessarily high level. However, in order to realize thedirect-current control method, a large torque is necessary in suddenacceleration at high speed rotation and, therefore, the current commandmay exceed the limit of the power amplifier to cause a so-called voltagesaturation so that the current is difficult to control.

In this case, the value of the integrator of the current loop increases.If the value of this integrator becomes excessively large, a maximumvoltage command is kept being outputted for a while, even after thecurrent command becomes smaller, so that an operation of the currentloop after the saturation of the voltage command would not be stable.

To cope with this problem, it has been a common practice to perform thefollowing saturation process. FIG. 12 shows D-phase and Q-phase controlsystems of a conventional AC servomotor, and FIG. 13 shows D-phase andQ-phase command voltages in the saturation process.

In FIG. 12, D-phase and Q-phase controllers are provided with anintegral term 11, 12 (K1 is an integral gain) and a proportional term13, 14 (K2 is a proportional gain), respectively, and the motor isrepresented by a resistance R and an inductance L. The D-phase andQ-phase controllers are provided with mutual interference terms 15, 16,respectively.

In FIG. 13, assuming that Vc represents a composite command voltage ofthe D-phase and Q-phase command voltages Vd, Vq and that Vdc representsa DC link voltage which is the maximum output voltage of the poweramplifier, the saturation process is performed in the following manner.

(1) The voltage command Vc is outputted as it is, in the relationshipVd² +Vq² ≦Vdc² (the vector of the composite command voltage Vc is withinor on a circle of the DC link voltage).

(2) The D-phase and Q-phase voltages Vd, Vq of the voltage command Vcare clamped in the following values, in the relationship Vd² +Vq² >Vdc²(the vector of the composite command voltage Vc is out of the circle ofthe DC link voltage).

    Vd=Vdc·Vd/(Vd.sup.2 +Vq.sup.2).sup.1/2            ( 2)

    Vq=Vdc·Vq/(Vd.sup.2 +Vq.sup.2).sup.1/2            ( 3)

And the values of the integrators are rewritten so that the outputs ofthe D-phase and Q-phase current controllers are the clamped D-phase andQ-phase voltages Vd, Vq, respectively.

Assuming that k1 represents an integral gain of the current loop, k2represents a proportional gain of the current loop, I represents atorque command and Ifb represents a current feedback, the voltagecommand Vc is expressed by the following equation (4).

    Vc=k1·(I-Ifb)/s-k2·Ifb                   (4)

From the equation (4), the maximum voltage command Vcmax set by theforegoing clamping is expressed by the following equation (5).

    Vcmax=k1·(I-Ifb)/s*-K2·Ifb               (5)

The integrator represented by 1/s* is set so that the current controlleroutputs the maximum voltage command Vcmax, and is expressed by thefollowing equation (6).

    1/s*=(Vcmax+k2·Ifb)/k1                            (6)

The integrators for the D-phase and Q-phase current control loops areexpressed as follows:

    D phase: 1/s*=(Vdmax+k2·dfb)/k1                   (6-1)

    Q phase: 1/s*=(Vqmax+k2·qfb)/k1                   (6-1)

By the saturation process of rewriting the integrators, the currentcontrollers output the clamped D-phase and Q-phase voltages to restrictthe composite voltage output Vc always within the DC link voltage Vdc.

However, in the conventional current control method in which thesaturation process is performed for both the D and Q phases insaturation of the voltage command, the acceleration characteristic wouldbe lowered.

In the current control using DQ conversion, the D-phase current Id inthe same orientation of magnetic flux φ is set to "0" and the Q-phasecurrent Iq perpendicular to the D-phase current Id is controlled so asto follow the torque command. Assuming that ω represents an angularspeed of the rotor, ω>0 and Iq>0 when the rotor is rotating forward andis being accelerated. The D-phase and Q-phase voltages at that time isshown in a vector diagram of FIG. 14.

When the composite vector voltage Vc of the D-phase and Q-phase commandvoltages Vd, Vq exceeds the voltage limit value Vlim (Vdc), thecomposite vector voltage Vc is converted into a voltage Vc' by changingits magnitude to Vlim with its phase unchanged so that the commandvoltages are clamped. This relation can be expressed by the followingequations.

    Vd=Vlim·sin θ                               (7)

    Vq=Vlim·cos θ                               (8)

With respect to the phase θ, there is a relation tan θ=Vd/Vq.

FIG. 15a shows the relation between the D-phase command voltage and thecurrent, and FIG. 15b shows the relation between the Q-phase commandvoltage and the current, when the composite voltage Vc does not exceedthe voltage limit value Vlim.

In the current control system by D-Q conversion, the D-phase currentcommand to flow an invalid current is set to "0" and the current controlis performed by the Q-phase current command. At that time, as is shownby the equation (1), a negative voltage (-ωL·Iq) is generated in the Dphase due to the voltage interference in the motor.

In the case where the composite voltage Vc does not exceed the voltagelimit value Vlim, the D-phase command voltage Vd reaches theinterference voltage (-ωL·Iq) as shown in FIG. 15a, so that no currentflows in the D phase. Therefore, any interference voltage due to theD-phase current does not appear in the Q phase.

When the D-phase and Q-phase command voltages are clamped as thecomposite command voltage Vc exceeds the voltage limit value Vlim, theD-phase command voltage Vd can not reach the interference voltage(-ωL·Iq), so that a positive current Id flows in the D phase as shown inFIG. 16a. This positive D-phase current Id adds (ωL·Iq) to the Q-phasevoltage Vq in FIG. 16b, thus increasing the Q-phase voltage Vq asexpressed by the following equation (9):

    Vq=ωφ+Z·Iq+ωL·Id         (9)

Namely, when the command voltages are clamped by voltage saturationduring acceleration, the D-phase voltage Vd is decreased and the Q-phasevoltage Vq is increased by the Q-phase current Iq, so that the phase θof the clamped composite voltage Vc decreases to the composite voltageVc" indicated by a dotted line in FIG. 14. If the phase θ is delayed asthe D-phase current Id increases, the generated torque decreases. Inother words, when the Q-phase voltage is clamped to the DC link voltageVdc, an adequate voltage for increasing the Q-phase current is difficultto obtain by the increase of the D-phase current Id, thus lowering theacceleration characteristic of the servomotor.

In FIG. 17 showing a relation between the D-phase and Q-phase voltages,when the current flows in the positive direction in the D phase duringacceleration, the vector voltage is shifted inwardly of the DC linkvoltage as indicated by a reference character A, so that the voltagesaturation is relieved. Contrary, when the current flows in the negativedirection in the D phase, the vector voltage is shifted outwardly of theDC link voltage as indicated by a reference character B, so that thevoltage saturation is promoted. As is understood from the equation (1),since the current flows usually in the negative direction in the D phasedue to the voltage interference during acceleration, the voltagesaturation is promoted.

SUMMARY OF THE INVENTION

It is an object of the present invention to increase an acceleratingtorque and also to stabilize a deceleration current control when avoltage command is saturated, in a current control of a servomotor usingDQ conversion.

According to the present invention, if the voltage command is saturated,a saturation process is performed to rewrite the value of an integratorin the D-phase current control loop only when the servomotor is indeceleration in the current control of the servomotor using the DQconversion. If a voltage command is saturated in acceleration, asaturation process is performed to rewrite only an integrator in theQ-phase current control loop. As a result, an accelerating torque isincreased and also the deceleration current control is stabilized whenthe voltage command is saturated.

A large torque is necessary to sharply accelerate a servomotor inhigh-speed rotation. In such event, if the vector sum of the D-phase andQ-phase voltage commands exceeds the voltage limit value of a poweramplifier to be in saturation, the output of the power amplifier isrestricted to a clamped voltage command so that the current control ofthe servomotor can not be performed.

To cope with this problem, when the servomotor is in acceleration, thesaturation process is performed for the Q phase but not for the D phase,to rewrite a value of an integrator in the Q-phase current control loop.The saturation process for the Q phase restricts the value of theintegrator in the current loop and prevents the maximum voltage commandfrom being continuously outputted when the current command decreases, soas to stabilize the deceleration current control of the current loopafter the voltage command is saturated. Further, as the saturationprocess is not performed for the D phase, the D-phase voltage isoutputted without restriction so as to control the D-phase current,which is produced by the voltage interference, to be zero and alsoincrease the accelerating torque by increasing the Q-phase current.

In deceleration, the saturation process is performed for both of the Dand Q phases to rewrite the integrators in the current control loops forthe D-phase and Q-phase. The value of the integrators of the currentloop are thus restricted not to increase and also the maximum voltagecommand is prevented from being continuously outputted when the currentcommand decrease, thus stabilizing the deceleration current control inthe current loop after the voltage command is saturated. Indeceleration, since the D-phase current produced by the voltageinterference acts in such a direction to decrease the Q-phase current,the saturation process is performed for the D phase since it is notnecessary to control the D-phase current by the D-phase voltage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart generally showing the current control method for aservomotor according to the present invention;

FIG. 2 is a block diagram showing D-phase and Q-phase control systemsfor an AC servomotor, for carrying out the method of the invention;

FIGS. 3a and 3b are diagrams for showing states of D-phase and Q-phasevoltages during acceleration according to the present invention;

FIGS. 3c and 3d are vector diagrams showing a relation between D-phasevoltage and the Q-phase voltage, according to the present invention andthe conventional current control method, respectively;

FIGS. 4a and 4b are diagrams for showing states of D-phase and Q-phasevoltages, when command voltages are not saturated during decelerationaccording to the present invention;

FIGS. 5a, 5b and 5c are diagrams for showing states of D-phase andQ-phase voltages when the command voltages are saturated duringdeceleration;

FIG. 6 is a block diagram showing a servo-motor control system forcarrying out the method of the invention;

FIG. 7 is a flowchart showing a current loop control process to beperformed in every predetermined period by a processor of a digitalservo circuit of FIG. 6;

FIG. 8 is a graph showing a response result according to theconventional current control method;

FIG. 9 is a graph showing a response result according to the currentcontrol method of the invention;

FIG. 10 is a block diagram of a conventional control system for an ACservomotor;

FIG. 11 is a block diagram showing a control system for controlling anAC servomotor by DQ conversion;

FIG. 12 is a block diagram showing conventional D-phase and Q-phasecontrol systems for an AC servomotor;

FIG. 13 is a vector diagram showing a saturation process of the commandvoltage;

FIG. 14 is a vector diagram showing D-phase and Q-phase voltages insaturation process;

FIGS. 15a and 15b are diagrams for showing D-phase and Q-phase voltageswhen a composite voltage Vc does not exceed a voltage limit;

FIGS. 16a and 16b show D-phase and Q-phase voltages when the compositevoltage Vc exceeds the voltage limit; and

FIG. 17 shows a relation between D-phase and Q-phase voltages in DQconversion.

BEST MODE FOR CARRYING OUT THE INVENTION

The general procedure of a current control method for a servomotor,according to the present invention will be described referring to theflowchart of FIG. 1. In Step T1, it is determined whether a voltagecommand is saturated. If the voltage command is not saturated, a currentloop process rather than a saturation process is performed. If thevoltage command is saturated, the procedure proceeds to Step T2 toperform a saturation process to rewrite an integrator based on thevoltage command clamped with respect to the Q phase.

In Step T3, it is determined whether the servomotor is controlled to beaccelerated. If the servomotor is to be accelerated, the current loopprocess is performed without performing the saturation process withrespect to the D phase. If the servomotor is not to be accelerated, theprocedure proceeds to Step T4 to perform the current loop process afterthe saturation process with respect to the D phase.

With the above procedure, it is possible to achieve two effects ofincreasing an acceleration torque and also stabilizing a current controlin deceleration when the voltage command is saturated.

FIG. 2 is a block diagram of a control system for an AC servomotor forcarrying out the present invention, as it is divided into D-phase andQ-phase control systems. In FIG. 2, D-phase and Q-phase controllers areprovided with an integral term 11, 12 (K1 is an integral gain) and aproportional term 13, 14 (K2 is a proportional gain), respectively, andthe motor is represented by a resistance R and an inductance L. TheD-phase and Q-phase controllers are provided with mutual interferenceterms 15, 16, respectively. The above arrangement of the control systemis identical with that of the conventional control system as shown inFIG. 12.

According to the current control of the invention, the D-phaseintegrator is rewritten only in deceleration by the saturation process,while the Q-phase integrator is rewritten in acceleration anddeceleration by the saturation process, to thereby increase theacceleration torque in acceleration and also stabilize the currentcontrol for deceleration when the voltage command is saturated inacceleration and deceleration.

First, an operation in acceleration will be described. In the currentcontrol system by D-Q conversion, when the D-phase current command Id toflow an invalid current is set to "0" and the current control isperformed based on the Q-phase current command Iq, the interferencevoltage (-ωL·Iq) is produced in D phase due to the Q-phase current Iqlikewise in FIG. 16, as indicated by the equation (1).

According to the current control of the present invention, when thecomposite voltage Vc exceeds the voltage limit Vlim as the D-phase andQ-phase command voltages increase, the saturation process is performedwith respect to the Q phase but no saturation process is performed withrespect to the D phase. With this process, the D-phase voltage Vd canreach (-ωL·Iq), as shown in FIG. 3a, to prevent the D phase current Idfrom flowing. Since the D-phase current Id does not flow, nointerference voltage is generated in the Q-phase voltage V. Thisrelation can be expressed by FIG. 3b and the following equation (10):

    Vq=ωφ+Z·Iq                              (10)

FIGS. 3c and 3d respectively show a relation between the D-phase andQ-phase voltages according to the current control method of the presentinvention and the conventional current control method. Comparing thepresent method with the conventional method with respect to the Q-phasevoltage, since no interference voltage (ω·L·Iq) is produced in D phasein FIG. 3c according to the present invention, it is possible to take acomponent by the Q-phase current in the clamped voltage larger than thatin FIG. 3d, according to the conventional method. Since the torque ofthe servomotor is proportional to the Q-phase current, an accelerationcharacteristic of the servomotor can be improved.

Next, the operation in deceleration will be described. It is assumedthat the motor is rotating forward in deceleration, i.e., ω>0 and Iq<0.There is a relation |Z·Iq|>|ωφ| during a low-speed rotation as shown inFIG. 4a, and therefore Vq<0, and there is a relation |Z·Iq|<|ωφ| duringa high-speed rotation as shown in FIG. 4b, and therefore Vq>0.

When the command voltage is not saturated, the D-phase command voltagereaches the interference voltage (-ωL·Iq), as shown in FIG. 5a, andtherefore Id=0. Accordingly, no interference voltage due to the D-phasecurrent Id is produced in the Q phase.

When the command voltage is saturated, the D-phase voltage Vd is clampedso that a negative D-phase current Id flows as shown in FIG. 5b, thusdecreasing the Q-phase voltage Vq as shown in FIG. 5c. According to theincrease of the D-phase current, the D-phase and Q-phase voltages Vd andVq decrease to eliminate the voltage saturation.

By rewriting the values of the respective integrators of the D-phase andQ-phase controllers, it is possible to prevent the D-phase current Idfrom increasing. For example, as expressed by equations (6-1) and (6-2),the value of the D-phase integrator is rewritten as (Vdmax+K2·Idfb)/k1,and the value of the Q-phase integrator is rewritten as(Vqmax+K2·Iqfb)/k1.

In FIG. 2, by rewriting the values of the D-phase and Q-phaseintegrators to be (Vdmax+K2·Idfb)/k1 and (Vqmax+K2·Iqfb)/k1,respectively in the voltage saturation, the outputs of the D-phase andQ-phase current controller are Vdmax and Vqmax, respectively, so thatthe saturation process for the integrators is performed and also thecommand voltage is clamped.

FIG. 6 is a block diagram of a servo-motor control system for carryingout the method of the present invention. The architecture of theservo-motor control system is identical with the conventional digitalservo control system and will therefore be generally described here. InFIG. 6, reference number 20 designates a computerized numerical controlunit (CNC); 21, a shared RAM; 22, a digital servo circuit having aprocessor (CPU), RON, RAM, etc.; 23, a power amplifier such as atransistor inverter; M, an AC servomotor; 24, an encoder for generatingpulses in response to rotation of the AC servomotor M; and 25, a rotorposition detector for detecting a rotor phase.

FIG. 7 is a flowchart of a current loop control process to be performedin every predetermined period by the processor of the digital servocircuit 22. The processor of the digital servo circuit 22 reads aposition command (or a speed command) from the numerical control unit(CNC) via the shared RAM 21 to perform a position loop process and aspeed loop process.

First, the processor reads a torque command Iq* outputted from the speedloop process (Step S1) and fetches a rotor phase θ from the rotorposition detector 25 (Step S2).

Then the processor fetches the actual currents Iu and Iv of U and Vphases, which are detected by a current detector (step S3), andcalculates D-phase and Q-phase currents Id, Iq by the DQ conversionusing the fetched U-phase and V-phase actual currents Iu, Iv and therotor phase θ (Step S4).

An ordinary current loop process (proportional-plus-integral control) isperformed to obtain a D-phase command voltage Vd, using the D-phasecurrent Id as a feedback current and the D-phase current command of "0".A current loop process is performed to obtain a Q-phase voltage commandVq, using the torque command read in Step S1 as the Q-phase currentcommand and the Q-phase current value Iq calculated in Step S4 as afeedback current (Step S5).

Then, it is determined whether a composite vector Vc of the D-phase andQ-phase command voltages Vd, Vq obtained in Step S5 exceeds a DC linkvoltage Vdc, which is the voltage limit value Vlim. Namely, it isdetermined whether the value of (Vd² +Vq²) is larger than the value ofVdc² (Step S6).

In the case where the composite vector Vc is not exceeding the DC linkvoltage Vdc, the procedure proceeds to Step S11 where the D-phase andQ-phase command voltages Vd, Vq calculated in Step S5 are converted byDQ conversion to obtain and output U-Phase, V-phase and Q-phase voltagecommand values (Step S11).

In Step S6, when the composite vector Vc exceeds the DC link voltageVdc, it is judged that the command voltage is saturated and aproportional-plus-integral control is performed to calculate a Q-phasevoltage Vq (Step S7). Then, the processor rewrite the value of theQ-phase integrator to be (Vqmax+K2·Iqfb)/k1, to perform a Q-phasesaturation process (Step S8).

Subsequently, it is determined whether the servomotor is in accelerationor deceleration based on the sign of ω·Iq (Step S9).

In the determination of Step S9, if the motor is rotating forward (ω>0)and the torque command Iq* is positive, or if the motor is rotatingbackward (ω<0) and the torque command Iq* is negative, it is judged thatthe motor is in acceleration, and the procedure proceeds to Step S11where U-phase, V-phase and W-phase voltage command values are obtained.

In the determination of Step S9, if the motor is rotating forward (ω>0)and the torque command Iq* is negative, or if the motor is rotatingbackward (ω<0) and the torque command Iq* is positive, it is judged thatthe motor is in deceleration and the processor rewrites the value of theD-phase integrator as (Vdmax+K2·Idfb)/k1, to perform a D-phasesaturation process (Step S10), and then the procedure proceeds to StepS11 where U-phase, V-phase and W-phase voltage command values areobtained.

In the case where the D-phase saturation process of Step S10 isperformed, the Q-phase saturation process is also performed.

The obtained D-phase and Q-phase voltages Vd, Vq are converted by DQconversion to calculate and output U-phase, V-phase and W-phase voltagecommand values (Steps S11, S12), to terminate the current loop processof one period.

FIGS. 8 and 9 show response results according to the conventional methodand the method of the present invention, respectively, in which the samespeed command is set such that the motor is accelerated stepwise from-1500 rpm to +5000 rpm. As shown in FIG. 8, when the saturation processis performed for both the D and Q phases according to the conventionalmethod, the response characteristic is of approximately 100 ms from-5000 rpm to +3000 rpm and then it takes approximately 600 ms to reach+5000 rpm as the torque drops.

Contrary, as shown in FIG. 9, when the saturation process is performedonly for the D phase not for the Q phase according to the presentinvention, the response characteristic is of approximately 100 ms from-5000 rpm to +3000 rpm likewise the conventional method, but it takesonly approximately 200 ms to reach +5000 rpm as the torque slightlydrops, thus improving the acceleration characteristic.

As described above, according to the present invention, it is possibleto increase the acceleration torque and also to stabilize the currentcontrol for deceleration when the voltage command is saturated, in anelectric current control of a servomotor using the D-Q conversion.

We claim:
 1. A method of controlling an electric current in a servomotorby performing a DQ conversion for converting a three-phase alternatingcurrent into a direct current of D- and Q- phases and a direct-currentvoltage of D- and Q- phases into a three-phase alternating-currentvoltage and issuing voltage commands to a power amplifier from D-phaseand Q-phase current controllers, comprising the steps of:(a) determiningwhether said voltage command is saturated over a maximum voltage whichsaid power amplifier is able to output; (b) determining whether theservomotor is in acceleration or in deceleration; and (c) rewriting anintegrator of the D-phase current controller when it is determined thatthe voltage command is saturated in said step (a) and it is determinedthat the servomotor is in deceleration in said step (b).
 2. A method ofcontrolling a current in a servomotor by issuing voltage commands to apower amplifier from D-phase and Q-phase current control devices by D-Qconversion, which converts an AC three-phase current into D-phase andQ-phase DC currents or converts D-phase and Q-phase DC voltages into anAC three-phase voltage, comprising the steps of:(a) determining whetherthe voltage command is saturated over a maximum voltage which the poweramplifier is able to output; (b) determining whether the servomotor isin acceleration or in deceleration; and (c) rewriting an integrator ofthe Q-phase current controller when it is determined that the voltagecommand is saturated in said step (a) and it is determined that theservomotor is in acceleration in said step (b).
 3. The currentcontrolling method for a servomotor according to claim 1, wherein saidstep (c) further includes a step of rewriting the integrator of theD-phase current controller so that the D-phase current controller issuesa voltage command to cause said power amplifier to output the maximumvoltage.
 4. The current controlling method for a servomotor according toclaim 2, wherein said step (c) further includes a step of rewriting theintegrator of the Q-phase current control device so that the Q-phasecurrent controller issues a voltage command to cause the power amplifierto output the maximum voltage.
 5. The current controlling method for aservomotor according to claim 1, wherein said step (a) further includesa step of determining whether a vector sum of the D-phase and Q-phasevoltage commands exceeds the maximum voltage of said power amplifier. 6.The current controlling method for a servomotor according to claim 1,wherein said step (b) further includes a step of determining whether theservomotor is in acceleration or deceleration based on a direction ofrotation of the servomotor and a sign of the Q-phase current command. 7.A servomotor control system for converting a three-phase alternatingcurrent into a direct current of D- and Q- phases and a direct-currentvoltage of D- and Q- phases into a three-phase alternating-currentvoltage and issuing voltage commands to a power amplifier, comprising:acircuit determining whether each of the voltage commands are saturatedover a maximum voltage which the power amplifier is able to output, andwhether the servomotor is in acceleration or deceleration; and acontroller rewriting an integrator of the D-phase current when it isdetermined that the voltage command is saturated in the circuit and itis determined that the servomotor is in deceleration.
 8. A servomotorcontrol system for converting a three-phase alternating current into adirect current of D- and Q- phases and a direct-current voltage of D-and Q- phases into a three-phase alternating-current voltage and issuingvoltage commands to a power amplifier, comprising:a circuit determiningwhether each of the voltage commands are saturated over a maximumvoltage which the power amplifier is able to output, and whether theservomotor is in acceleration or deceleration; and a controllerrewriting an integrator of the Q-phase current when it is determinedthat the voltage command is saturated in the circuit and it isdetermined that the servomotor is in acceleration.
 9. The currentcontrolling method for a servomotor according to claim 2, wherein saidstep (a) further includes a step of determining whether a vector sum ofthe D-phase and Q-phase voltage commands exceeds the maximum voltage ofsaid power amplifier.
 10. The current controlling method for aservomotor according to claim 2, wherein said step (b) further includesa step of determining whether the servomotor is in acceleration ordeceleration based on a direction of rotation of the servomotor and asign of the Q-phase current command.